1. Field of the Invention
The present invention is related to a brushless dc motor and more particularly to the brushless dc motor having dual Hall sensors adapted to detect two permanent magnets of a rotor, so that the motor is capable of both alternatively changing the magnetic field of a stator and outputting a predetermined frequency specification of the motor speed.
2. Description of the Related Art
A conventional brushless dc fan motor outputs its rotational frequency to a computer system for reference thereby adjusting the motor speed to be increased or decreased. In outputting rotational frequency, Hall voltage signals inputted from a Hall sensor of the motor are adapted to be a reference for the rotational frequency. In design, the permanent magnet of the motor rotor is limited due to the pole count of the motor stator, four or eight poles for example. Thus, as the Hall sensor detects the speed fo the motor rotor the rotational frequency is determined by the pole count.
To output a predetermined specific rotational frequency applied to the computer system, the motor controller must include a frequency divider so that the motor of various pole counts is capable of outputting a predetermined specific rotational frequency of the motor speed.
Referring to FIG. 1, a circuitry diagram of a conventional brushless dc motor mainly includes a driver circuit 10, a sensor 11, a coil 12, and a frequency divider IC 13. Referring to FIG. 2, a motor comprises a rotor 14, a stator 15, and a driver member 16. The rotor 14 comprises a permanent magnet 141 surrounding an outer circumference of the stator 15, and a shaft 142 axially combined with the stator 15. The stator 15 comprises an axial tube 151 connected to the driver member 16 which is comprised of the driver circuit 10 and the frequency divider IC 13, as shown in FIG. 1. The driver member 16 further comprise the sensor 11 electrically connected to the driver circuit 10 which may actuate the coil 12 alternatively according to the signals of the sensor 11. Connected to the sensor 11, the frequency divider IC 13 is adapted to convert the signals of the sensor 11 into output of a predetermined specific frequency available for a computer system.
However, the problem is that the frequency divider IC is more expensive and increases overall cost of the motor. There is a need for an inexpensive member to replace the frequency divider IC.
The present invention intends to provide a brushless dc motor utilizing inexpensive members of a sensor associated with a frequency permanent magnet to replace an expensive member of frequency divider IC in such a way to mitigate and overcome the above problem.
The primary objective of the this invention is to provide a brushless dc motor having dual Hall sensors, which are inexpensive, to replace the conventional frequency divider IC, so that the overall cost of the motor is reduced.
The secondary objective of this invention is to provide the brushless dc motor having dual Hall sensors associated with a driver permanent magnet and a frequency permanent magnet respectively, which is adapted to drive the motor and generate a predetermined specific rotational frequency available for a computer system by detecting alternation of the opposite magnetic poles of the respective driver and frequency permanent magnets.
The brushless dc motor in accordance with the present invention mainly comprises a stator, a first Hall sensor, a second Hall sensor, and a rotor. The stator, which comprises a plurality of magnet poles, is axially combined with the rotor which comprises a driver permanent magnet and a frequency permanent magnet. The driver permanent magnet is adapted to rotate the rotor by means of being detected by the first Hall sensor while the frequency permanent magnet with predetermined pole count adapted to generate the rotational frequency of the rotor by means of being detected by the second Hall sensor.
Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description and the accompanying drawings.